The present invention relates generally to a zoom lens apparatus and more particularly to a zoom lens apparatus having a distance adjustment mechanism for a first and a second lens group.
Conventional zoom lens apparatus generally have two movable lens groups. The first lens group is generally used for magnification purposes and the second lens group is used for focus correction. The distance between the two lens groups from the film surface is adjusted to achieve the zooming effect. The group 1 lens element is used for focal length adjustment. The group 2 lens element is used for focus correction, and is positioned between the group 1 lens element and the film surface. To achieve the zooming effect, the group 1 lens element is moved farther away from the film surface by a distance X. For refocusing the image onto the film, the group 2 lens element is also moved farther away from the film surface by a distance Y. To achieve proper focus, distance Y is larger than distance X, the exact relationship being readily derived from well known optics principles such as Snell""s law. In order to maintain the ratio between distances X and Y, many different cam and gear mechanisms have been designed in the art. Some conventional cameras provide a cam mechanism having a cam barrel with two independent helicoid cam grooves with different gradients for the movement of the two lens groups. The first cam groove is used for moving the first lens group, while the second cam grove is used for moving the second lens group. Since the distance Y is greater than X, the gradient (defined as the angle formed between the traverse-sectional edge of the groove barrel and the groove) for the second groove is larger (steeper) than the first groove. The steeper gradient results in poor accuracy and less reliable mechanical movement for the group 2 lens element. There is therefore a need to provide an improved lens movement mechanism for zoom lenses.
Accordingly, the present invention provides a zoom lens assembly that couples the second lens group to the first lens group such that one cam mechanism is able to transfer both lens groups. The second lens group is further provided with a second cam mechanism that is able to provide the fine distance adjustment of the second lens group required for proper focus correction.
The zoom lens assembly according to the present invention contains a first lens group with a first lens defining an optical axis. A first cam mechanism, adapted for mounting onto a camera, is coupled to the first lens group for moving the first lens group along the optical axis. A second lens is provide within the second lens group and has an optical axis that is aligned with the optical axis of the first lens. The second lens group is coupled to the first cam mechanism such that moving the first lens group along the optical axis by operating the first cam mechanism translates into a movement of the second lens group. In addition, a second cam mechanism is used to further couple the first cam mechanism and the second lens group such that operating the first cam mechanism causes an additional movement of the second lens group along the same optical axis.
In the preferred embodiment, the first lens group is attached inside a first cam barrel such that the optical axis coincide with the longitudinal axis of the first cam barrel. A rotary barrel, adapted for rotatable attachment onto the mount opening of a camera, is slidably fitted over the first cam barrel and is provided with a cam drive to slidably engage a helicoid cam groove cut on the first cam barrel.
In the preferred embodiment, the second lens group is fitted within a second cam barrel which contains one or more helicoid channels and one or more longitudinal channels. The second lens group is also attached to the first cam barrel in a manner such that there is no rotational movement of the second lens group relative to the first cam barrel. The cam drive engages to the longitudinal channel such that rotating the rotary barrel causes a corresponding rotation of a second cam barrel. At least one second cam drive is mounted on the second lens group and interacts slidably with the helicoid channel such that the rotational twist of the rotary barrel by the user is translated into additional longitudinal movement of the second lens group.
An important result of this combination cam mechanism is that the helicoid channel that is cut into the second cam barrel does not need to have a steep gradient, because the purpose of the helicoid channel is only for the fine positioning required for focus correction of the second lens group. The larger travelling distance that is required for focal length adjustment for both lens groups is rendered by the helicoid groove of the first cam barrel. As a result, the helicoid channel has a shallow gradient that gives greater stability and the reliability.